Duct CFM Calculator

Estimate total system airflow from tonnage (~400 CFM/ton) and a room's share of it by area.

Inputs
tons
sq ft
sq ft
Results update as you type — no submit
ResultsLIVE
Total system airflow
1,200 CFM
≈ 2,039 m³/h
This room's share120 CFM · 204 m³/h
Airflow per ton400 CFM/ton
First-pass split by area — rooms with heavy sun or glass need more. A room-by-room Manual J / Manual D duct design, balanced with dampers, is definitive.
Formula
total = 3 × 400 = 1,200 CFM; room = × (150/1,500) = 120 CFM
How it works

Ductwork is sized around airflow, measured in CFM — cubic feet of air per minute. The starting rule for a residential system is about `400` CFM per ton of cooling capacity, so a `3`-ton system moves roughly `1,200` CFM total. Humid climates often design a little lower, near `350` CFM/ton, to give the coil more time to wring out moisture; dry climates run higher, near `450`.

Once you know total airflow, a first-pass way to split it among rooms is by floor area: a room gets the share of total CFM equal to its share of the conditioned square footage. A `150` square-foot bedroom in a `1,500` square-foot home is `10%` of the area, so it gets about `10%` of `1,200` CFM, or `120` CFM. That sets a target for the supply register and branch duct feeding that room.

This area split is a planning estimate, not a duct design. Real rooms differ in heat load — a west-facing room with big windows needs more air than a shaded interior room of the same size — so a proper job uses a room-by-room Manual J load calculation feeding a Manual D duct design, then balancing dampers to fine-tune. Use these numbers to sanity-check registers and spot rooms that are starved for air.

Worked example
System `3` tons · `400` CFM/ton · room `150` sq ft of `1,500` sq ft home
total airflow = 3 × 400 = `1,200 CFM` (`2,039 m³/h`)
room airflow = 1,200 × (150 ÷ 1,500) = `120 CFM`
Questions about duct CFM
About 400 CFM per ton is the standard residential design point, with a range of 350–450. Use 350 in humid climates that need more dehumidification, 400 as a default, and 450 in dry climates. A 4-ton system at 400 CFM/ton moves 1,600 CFM.
SOURCES — ACCA Manual D residential duct design · ~400 CFM/ton airflow convention
Rule-of-thumb estimate — a Manual J load calculation by a licensed HVAC pro is definitive
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